Devices for pneumatic control are used in particular in the manufacture of blow-molded containers to be able to achieve delivery of one or more blow pressures that is coordinated with execution of the blow molding process.
In this type of container molding using blow pressure action, preforms of a thermoplastic material, for example preforms of PET (polyethylene terephthalate), are delivered to different processing stations within a blow molding machine. Typically, such a blow molding machine has a heater unit and a blow unit, in the vicinity of which the previously temperature treated preform is expanded by biaxial orientation to a container. The expansion is accomplished with the use of compressed air, which is introduced into the preform to be expanded. The process sequence of such an expansion is explained in DE-OS 43 40 291.
The basic structure of a blow station for molding containers is described in DE-OS 42 12 583. Options for temperature treatment of the preforms are explained in DE-OS 23 52 926.
Within the device for blow molding, the preforms and the blow-molded containers can be transported by means of various handling devices. One proven technique in particular is the use of transport mandrels onto which the preforms are placed. The preforms can also be handled with other carrying devices, however. For example, the use of grippers for handling preforms, and the use of expansion mandrels that can be introduced into a mouth area of the preform for the purpose of holding, are also among the available designs.
The aforementioned handling of the preforms takes place on the one hand as part of the so-called two-stage process, in which the preforms are first manufactured in an injection molding process, then are stored temporarily, before later being conditioned with respect to their temperature and blow molded into containers. On the other hand, application is also found in the so-called one-stage process, in which the preforms are appropriately temperature treated and then blow molded immediately after their production by injection molding and adequate hardening.
With regard to the blow stations employed, various embodiments are known. In blow stations that are arranged on rotating transport wheels, one frequently encounters mold supports that swing open in a book-like fashion. However, it is also possible to use mold supports that slide relative to one another or operate in other ways. In stationary blow stations, which are especially suitable for accommodating multiple cavities for container molding, plates that typically are arranged parallel to one another are used as molds.
The devices for pneumatic control used to supply compressed air to the blow stations are typically implemented as high pressure valves in which a control piston is positioned by a control pressure in such a way that the control piston either closes or opens a flow path for the blow pressure. To ensure a proper seal, the known control pistons are usually vulcanized with an elastomer in the area of a seal face.
A particular disadvantage of the known control pistons is that, because of the metallic construction of the control piston, it is necessary to avoid direct contact with the wall of the surrounding cylinder, since otherwise severe wear would occur. To avoid this contact, guide bands of plastic or similar soft materials are used, which are employed as guide elements, similar to the arrangement of piston rings. For these guide bands, appropriate recesses must be placed in the region of the piston wall, resulting in corresponding manufacturing costs. Moreover, installation of the guide bands also has a corresponding cost.